PROVENANCE ANALYSIS AND DEPOSITIONAL SYSTEM OF THE LATE QUATERNARY SEDIMENT FROM THE GANGES-BRAHMAPUTRA (G-B) DELTA, BANGLADESH: APPLICATION OF STRONTIUM GEOCHEMISTRY

The Ganges and Brahmaputra River, which drain the crystalline rocks of the High Himalayas, Lesser Himalayas and Tibetan Sedimentary Series, have transported the major portion of late Quaternary sediments to the Ganges - Brahmaputra (G-B) delta, Bangladesh. These major lithologic groups also have distinctive Sr concentrations and isotopic ratios that are used in this study to understand the late Quaternary river avulsion and abandonment, and shifting provenance sources for the G-B delta. It is found from the present study that, the Pleistocene sediment in the entire G-B delta are geochemically different from the Holocene because they have completely different source areas and transported by two different river systems. Sr concentrations and isotopic ratios of the Pleistocene sediment collected from 12 boreholes across the G-B delta are well above 150 ppm and less than 0.73 respectively indicating transport by the Brahmaputra River alone mixing sediment from the Tibetan Plateau and Higher Himalaya. In contrast, the Holocene sediment collected from the same boreholes show a wide variation in Sr concentrations and isotopic ratios increasing and decreasing respectively from west to east. Based on Sr concentrations and isotopic ratios of the Holocene sediment, the G-B delta can be divided into three deltaic zones - the eastern, central, and western delta. The western delta contains Ganges input alone from the Higher Himalaya; the central delta is a mixing zone between the Ganges and Brahmaputra sediment and in the eastern delta, Holocene sediments were deposited only by the Brahmaputra. The Sr concentrations and isotopic ratios of these borehole sediment at different depths from all over the G-B delta supports a model where the Brahmaputra was the principal source of sediment deposited in the delta during the Late Pleistocene. Decreasing Sr concentrations in the Early Holocene sediment in the western delta suggest that the Ganges River avulsed from the West Bengal of India into the western G-B delta in Bangladesh. The avulsion of the Ganges is associated with a similar shift of the Brahmaputra to the east, possibly indicating a basin wide reorganization of the river channels. By the late Holocene, the Ganges and Brahmaputra Rivers and the G-B delta had developed their present course and delta morphology.